When their right hand has a red spot from it laying on the mousepad too long and their left hand has a red line from it laying on the edge of the keyboard too long, usually a sign that they have been playing an rpg game too much.

Guy 1: ...Hey. *tired*
Guy 2: You look horrible! Have you been up all night?
Guy 1: No...
Guy 2: Let me see your hands. Yep, you have the mark of the geek, you've been up playing that rpg game!